Health and Human Services Secretary Kathleen Sebelius is stepping down. The departure of one of the president's longest serving cabinet secretaries comes just over six months after the botched rollout of the government's new health insurance website. President Obama already has a replacement selected. This morning he's expected to nominate Sylvia Mathews Burwell to take over the health department and also the ongoing challenges of implementing the Affordable Care Act.

NPR's Scott Horsley joins us now to talk about the shake-up. Morning, Scott.

SCOTT HORSLEY, BYLINE: Good morning, David.

GREENE: So Scott, interesting timing here. I mean, a lot of people were calling for Sebelius to step down, you know, for awhile after the rollout of Obamacare, but the last few weeks the news about the website has been pretty positive. So why now?

HORSLEY: Well, it really has been pretty positive in recent weeks. After that disastrous debut, the White House brought in a team of outside technical experts to fix the website and they really succeeded. Even with that botched rollout, the administration actually exceeded its signup targets. Secretary Sebelius and others barnstormed the country promoting the healthcare exchanges, and just yesterday the secretary was on Capitol Hill telling lawmakers the number of people signing up for private insurance by the exchanges tops 7.5 million people.

KATHLEEN SEBELIUS: During these past six months, millions have obtained the security and peace of mind of affordable health coverage. Many of the people I've met have told me that they've been able to get coverage for the first time in years and some have insurance for the first time in their entire lives.

HORSLEY: So she's able to go out on something of a high note, but that stumble out of the gate really did damage to Sebelius' reputation and the president's.

GREENE: Well, can she be blamed for that stumble out of the gate?

HORSLEY: Well, the president himself has said, you know, Sebelius was not the one hunched over the computer writing software code and so her fingerprints are not on all those frustrating bugs, but that said, she and the people underneath her obviously failed to ride herd on the government contractors who were writing the software, and what's more, they failed to sound the alarm about just how bad the problems were.

That left the president very exposed because as late as September, he was out there boasting that this was going to be as easy to use as Amazon and Kayak. In addition, Sebelius didn't really help with some of her own public appearances early on. Jon Stewart used her as a punching bag and a punchline when she appeared on his program in the fall, back when the website was barely functioning.

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GREENE: I had forgotten about that.

HORSLEY: Yeah. So on hand you can point to the repair of the website and those 7.5 million sign-ups as a remarkable comeback. On the other hand, you can't help thinking, you know, if management had been on top of things, if the fix-it crew had been brought in in a more timely manner, a huge black eye could have been avoided.

GREENE: Well, I mean in a way all of this is a little surprising because Kathleen Sebelius came into this job with really solid credentials.

HORSLEY: She had executive experience. She'd been a successful governor of Kansas. She was also that state's insurance commissioner so she understands the healthcare market. One of the things the administration has said is they were so focused on making sure these new insurance supermarkets were stocked with competitive policies, they failed to pay enough attention to whether customers would be able to get out through the checkout line.

One other thing to say about the secretary is that she was one of the strong advocates inside the administration for making sure insurance policies included access to birth control. That's a requirement that's put the White House at odds with the Catholic Church and it's also the subject of a couple legal challenges now pending before the Supreme Court.

GREENE: And Scott, what can you tell us about the nominee to replace Sebelius, Sylvia Mathews Burwell?

HORSLEY: She is a veteran of the Clinton administration and she also worked for the Gates Foundation and the Wal-Mart foundation. She was confirmed unanimously by the U.S. Senate in her current job as budget director, and perhaps not surprisingly the White House is really touting her as someone with strong credentials as a manager.